Nixpkgs User’s Guide¶
How to install Haskell packages¶
Nixpkgs distributes build instructions for all Haskell packages registered on Hackage, but strangely enough normal Nix package lookups don’t seem to discover any of them, except for the default version of ghc, cabal-install, and stack:
$ nix-env -i alex
error: selector ‘alex’ matches no derivations
$ nix-env -qa ghc
ghc-8.10.2
The Haskell package set is not registered in the top-level namespace
because it is huge. If all Haskell packages were visible to these
commands, then name-based search/install operations would be much slower
than they are now. We avoided that by keeping all Haskell-related
packages in a separate attribute set called haskellPackages
, which
the following command will list:
$ nix-env -f "<nixpkgs>" -qaP -A haskellPackages
haskellPackages.a50 a50-0.5
haskellPackages.AAI AAI-0.2.0.1
haskellPackages.abacate abacate-0.0.0.0
haskellPackages.abc-puzzle abc-puzzle-0.2.1
haskellPackages.abcBridge abcBridge-0.15
haskellPackages.abcnotation abcnotation-1.9.0
haskellPackages.abeson abeson-0.1.0.1
[... some 14000 entries omitted ...]
To install any of those packages into your profile, refer to them by their attribute path (first column):
nix-env -f "<nixpkgs>" -iA haskellPackages.Allure ...
The attribute path of any Haskell packages corresponds to the name of
that particular package on Hackage: the package cabal-install
has
the attribute haskellPackages.cabal-install
, and so on. (Actually,
this convention causes trouble with packages like 3dmodels
and
4Blocks
, because these names are invalid identifiers in the Nix
language. The issue of how to deal with these rare corner cases is
currently unresolved.)
Haskell packages whose Nix name (second column) begins with a
haskell-
prefix are packages that provide a library whereas packages
without that prefix provide just executables. Libraries may provide
executables too, though: the package haskell-pandoc
, for example,
installs both a library and an application. You can install and use
Haskell executables just like any other program in Nixpkgs, but using
Haskell libraries for development is a bit trickier and we’ll address
that subject in great detail in section How to create a development
environment.
Attribute paths are deterministic inside of Nixpkgs, but the path
necessary to reach Nixpkgs varies from system to system. We dodged that
problem by giving nix-env
an explicit -f "<nixpkgs>"
parameter,
but if you call nix-env
without that flag, then chances are the
invocation fails:
$ nix-env -iA haskellPackages.cabal-install
error: attribute ‘haskellPackages’ in selection path
‘haskellPackages.cabal-install’ not found
On NixOS, for example, Nixpkgs does not exist in the top-level namespace by default. To figure out the proper attribute path, it’s easiest to query for the path of a well-known Nixpkgs package, i.e.:
$ nix-env -qaP coreutils
nixos.coreutils coreutils-8.23
If your system responds like that (most NixOS installations will), then
the attribute path to haskellPackages
is nixos.haskellPackages
.
Thus, if you want to use nix-env
without giving an explicit -f
flag, then that’s the way to do it:
nix-env -qaP -A nixos.haskellPackages
nix-env -iA nixos.haskellPackages.cabal-install
Our current default compiler is GHC 8.10.x and the haskellPackages
set contains packages built with that particular version. Nixpkgs
contains the last three major releases of GHC and there is a whole
family of package sets available that defines Hackage packages built
with each of those compilers, too:
nix-env -f "<nixpkgs>" -qaP -A haskell.packages.ghc8104
nix-env -f "<nixpkgs>" -qaP -A haskell.packages.ghc901
The name haskellPackages
is really just a synonym for
haskell.packages.ghcXYZ
(where XYZ
is current default GHC
version in Nixpkgs), because we prefer that package set
internally and recommend it to our users as their default choice, but
ultimately you are free to compile your Haskell packages with any GHC
version you please. The following command displays the complete list of
available compilers:
$ nix-env -f "<nixpkgs>" -qaP -A haskell.compiler
haskell.compiler.ghc8102Binary ghc-8.10.2-binary
haskell.compiler.ghc8102BinaryMinimal ghc-8.10.2-binary
haskell.compiler.ghc8104 ghc-8.10.4
haskell.compiler.integer-simple.ghc8104 ghc-8.10.4
haskell.compiler.ghcHEAD ghc-8.11.20200824
haskell.compiler.native-bignum.ghcHEAD ghc-8.11.20200824
haskell.compiler.ghc865Binary ghc-8.6.5-binary
haskell.compiler.ghc884 ghc-8.8.4
haskell.compiler.integer-simple.ghc884 ghc-8.8.4
haskell.compiler.ghc901 ghc-9.0.1
haskell.compiler.integer-simple.ghc901 ghc-9.0.1
We have no package sets for jhc
or uhc
yet, unfortunately, but
for every version of GHC listed above, there exists a package set based
on that compiler. Also, the attributes haskell.compiler.ghcXYZ
and
haskell.packages.ghcXYZ.ghc
are synonymous for the sake of
convenience.
How to install a branch of a package¶
One of the nice things about Nix is that nixpkgs contains all information needed to build a package. This makes it easy to point a package to a different branch of the source and have Nix build a package for that branch.
Even though Haskell packages are typically generated based on the hackage
releases, because hackage contains source packages this is still possible
for hackage. You can use overrideSrc
to override the source, for example:
my-hledger-lib = (haskell.lib.overrideSrc haskellPackages.hledger-lib {
src = /home/aengelen/dev/hledger/hledger-lib;
});
my-hledger = (haskell.lib.overrideSrc haskellPackages.hledger {
src = /home/aengelen/dev/hledger/hledger;
}).override {
hledger-lib = my-hledger-lib;
};
hledger-web = haskell.lib.justStaticExecutables ((haskell.lib.overrideSrc haskellPackages.hledger-web {
src = /home/aengelen/dev/hledger/hledger-web;
})
.override {
hledger = my-hledger;
hledger-lib = my-hledger-lib;
});
How to create a development environment¶
How to install a compiler¶
A simple development environment consists of a Haskell compiler and one
or both of the tools cabal-install
and stack
. We saw in section
How to install Haskell packages
how you can install those programs into your user profile:
nix-env -f "<nixpkgs>" -iA haskellPackages.ghc haskellPackages.cabal-install
Instead of the default package set haskellPackages
, you can also use
the more precise name haskell.compiler.ghc7102
, which has the
advantage that it refers to the same GHC version regardless of what
Nixpkgs considers “default” at any given time.
Once you’ve made those tools available in $PATH
, it’s possible to
build Hackage packages the same way people without access to Nix do it
all the time:
cabal get lens-4.11 && cd lens-4.11
cabal install -j --dependencies-only
cabal configure
cabal build
If you enjoy working with Cabal sandboxes, then that’s entirely possible too: just execute the command
cabal sandbox init
before installing the required dependencies.
The nix-shell
utility makes it easy to switch to a different
compiler version; just enter the Nix shell environment with the command
nix-shell -p haskell.compiler.ghc784
to bring GHC 7.8.4 into $PATH
. Alternatively, you can use Stack
instead of nix-shell
directly to select compiler versions and other
build tools per-project. It uses nix-shell
under the hood when Nix
support is turned on. See How to build a Haskell project using
Stack.
If you’re using cabal-install
, re-running cabal configure
inside
the spawned shell switches your build to use that compiler instead. If
you’re working on a project that doesn’t depend on any additional system
libraries outside of GHC, then it’s even sufficient to just run the
cabal configure
command inside of the shell:
nix-shell -p haskell.compiler.ghc784 --command "cabal configure"
Afterwards, all other commands like cabal build
work just fine in
any shell environment, because the configure phase recorded the absolute
paths to all required tools like GHC in its build configuration inside
of the dist/
directory. Please note, however, that
nix-collect-garbage
can break such an environment because the Nix
store paths created by nix-shell
aren’t “alive” anymore once
nix-shell
has terminated. If you find that your Haskell builds no
longer work after garbage collection, then you’ll have to re-run
cabal configure
inside of a new nix-shell
environment.
How to install a compiler with libraries¶
GHC expects to find all installed libraries inside of its own lib
directory. This approach works fine on traditional Unix systems, but it
doesn’t work for Nix, because GHC’s store path is immutable once it’s
built. We cannot install additional libraries into that location. As a
consequence, our copies of GHC don’t know any packages except their own
core libraries, like base
, containers
, Cabal
, etc.
We can register additional libraries to GHC, however, using a special
build function called ghcWithPackages
. That function expects one
argument: a function that maps from an attribute set of Haskell packages
to a list of packages, which determines the libraries known to that
particular version of GHC. For example, the Nix expression
ghcWithPackages (pkgs: [pkgs.mtl])
generates a copy of GHC that has
the mtl
library registered in addition to its normal core packages:
$ nix-shell -p "haskellPackages.ghcWithPackages (pkgs: [pkgs.mtl])"
[nix-shell:~]$ ghc-pkg list mtl
/nix/store/zy79...-ghc-7.10.2/lib/ghc-7.10.2/package.conf.d:
mtl-2.2.1
This function allows users to define their own development environment
by means of an override. After adding the following snippet to
~/.config/nixpkgs/config.nix
,
{
packageOverrides = super: let self = super.pkgs; in
{
myHaskellEnv = self.haskell.packages.ghc7102.ghcWithPackages
(haskellPackages: with haskellPackages; [
# libraries
arrows async cgi criterion
# tools
cabal-install haskintex
]);
};
}
it’s possible to install that compiler with
nix-env -f "<nixpkgs>" -iA myHaskellEnv
. If you’d like to switch
that development environment to a different version of GHC, just replace
the ghc7102
bit in the previous definition with the appropriate
name. Of course, it’s also possible to define any number of these
development environments! (You can’t install two of them into the same
profile at the same time, though, because that would result in file
conflicts.)
The generated ghc
program is a wrapper script that re-directs the
real GHC executable to use a new lib
directory — one that we
specifically constructed to contain all those packages the user
requested:
$ cat $(type -p ghc)
#! /nix/store/xlxj...-bash-4.3-p33/bin/bash -e
export NIX_GHC=/nix/store/19sm...-ghc-7.10.2/bin/ghc
export NIX_GHCPKG=/nix/store/19sm...-ghc-7.10.2/bin/ghc-pkg
export NIX_GHC_DOCDIR=/nix/store/19sm...-ghc-7.10.2/share/doc/ghc/html
export NIX_GHC_LIBDIR=/nix/store/19sm...-ghc-7.10.2/lib/ghc-7.10.2
exec /nix/store/j50p...-ghc-7.10.2/bin/ghc "-B$NIX_GHC_LIBDIR" "$@"
The variables $NIX_GHC
, $NIX_GHCPKG
, etc. point to the new
store path ghcWithPackages
constructed specifically for this
environment. The last line of the wrapper script then executes the real
ghc
, but passes the path to the new lib
directory using GHC’s
-B
flag.
The purpose of those environment variables is to work around an impurity
in the popular
ghc-paths library.
That library promises to give its users access to GHC’s installation
paths. Only, the library can’t possible know that path when it’s
compiled, because the path GHC considers its own is determined only much
later, when the user configures it through ghcWithPackages
. So we
patched
ghc-paths
to return the paths found in those environment variables at run-time
rather than trying to guess them at compile-time.
To make sure that mechanism works properly all the time, we recommend
that you set those variables to meaningful values in your shell
environment, too, i.e. by adding the following code to your
~/.bashrc
:
if type >/dev/null 2>&1 -p ghc; then
eval "$(egrep ^export "$(type -p ghc)")"
fi
If you are certain that you’ll use only one GHC environment which is
located in your user profile, then you can use the following code, too,
which has the advantage that it doesn’t contain any paths from the Nix
store, i.e. those settings always remain valid even if a nix-env -u
operation updates the GHC environment in your profile:
if [ -e ~/.nix-profile/bin/ghc ]; then
export NIX_GHC="$HOME/.nix-profile/bin/ghc"
export NIX_GHCPKG="$HOME/.nix-profile/bin/ghc-pkg"
export NIX_GHC_DOCDIR="$HOME/.nix-profile/share/doc/ghc/html"
export NIX_GHC_LIBDIR="$HOME/.nix-profile/lib/ghc-$($NIX_GHC --numeric-version)"
fi
How to install a compiler with libraries, hoogle and documentation indexes¶
If you plan to use your environment for interactive programming, not
just compiling random Haskell code, you might want to replace
ghcWithPackages
in all the listings above with ghcWithHoogle
.
This environment generator not only produces an environment with GHC and
all the specified libraries, but also generates a hoogle
and
haddock
indexes for all the packages, and provides a wrapper script
around hoogle
binary that uses all those things. A precise name for
this thing would be
“ghcWithPackagesAndHoogleAndDocumentationIndexes
”, which is,
regrettably, too long and scary.
For example, installing the following environment
{
packageOverrides = super: let self = super.pkgs; in
{
myHaskellEnv = self.haskellPackages.ghcWithHoogle
(haskellPackages: with haskellPackages; [
# libraries
arrows async cgi criterion
# tools
cabal-install haskintex
]);
};
}
allows one to browse module documentation index not too dissimilar to
this
for all the specified packages and their dependencies by directing a
browser of choice to ~/.nix-profile/share/doc/hoogle/index.html
(or
/run/current-system/sw/share/doc/hoogle/index.html
in case you put
it in environment.systemPackages
in NixOS).
After you’ve marveled enough at that try adding the following to your
~/.ghc/ghci.conf
:def hoogle \s -> return $ ":! hoogle search -cl --count=15 \"" ++ s ++ "\""
:def doc \s -> return $ ":! hoogle search -cl --info \"" ++ s ++ "\""
and test it by typing into ghci
:
:hoogle a -> a
:doc a -> a
Be sure to note the links to haddock
files in the output. With any
modern and properly configured terminal emulator you can just click
those links to navigate there.
Finally, you can run
hoogle server --local -p 8080
and navigate to http://localhost:8080/ for your own local
Hoogle. The --local
flag makes
the hoogle server serve files from your nix store over http, without the
flag it will use file://
URIs. Note, however, that Firefox and
possibly other browsers disallow navigation from http://
to
file://
URIs for security reasons, which might be quite an
inconvenience. Versions before v5 did not have this flag. See this
page for
workarounds.
For NixOS users there’s a service which runs this exact command for you.
Specify the packages
you want documentation for and the
haskellPackages
set you want them to come from. Add the following to
configuration.nix
.
services.hoogle = {
enable = true;
packages = (hpkgs: with hpkgs; [text cryptonite]);
haskellPackages = pkgs.haskellPackages;
};
How to install haskell-language-server¶
In short: Install pkgs.haskell-language-server
and use the
haskell-language-server-wrapper
command to run it. See the
hls user guide
on how to configure your text editor to use hls and how to test your setup.
Hls needs to be compiled with the ghc version of the project you use it on.
pkgs.haskell-language-server
provides
haskell-language-server-wrapper
, haskell-language-server
and haskell-language-server-x.x.x
binaries, where x.x.x
is the ghc version for which it is compiled.
By default it includes binaries for all ghc versions that are provided
in the binary caches. You can override that list with e.g.
pkgs.haskell-language-server.override { supportedGhcVersions = [ "884" "901" ]; }
When you run haskell-language-server-wrapper
it will detect the ghc
version used by the project you are working on (by asking e.g. cabal or
stack) and pick the appropriate above mentioned binary from your path.
Be careful when installing hls globally and using a pinned nixpkgs for a
Haskell project in a nix-shell. If the nixpkgs versions deviate to much
(e.g. use different glibc
versions) hls might fail. It is
recommended to then install hls in the nix-shell from the nixpkgs
version pinned in there.
If you know, that you only use one ghc version, e.g. in a project
specific nix-shell You can either use an override as given above or
simply install pkgs.haskellPackages.haskell-language-server
instead
of the top-level attribute pkgs.haskell-language-server
.
How to make haskell-language-server find a GHC from nix-shell¶
If you use nix-shell for your development environments then haskell-language-server
will not find an installed GHC or will find a GHC with an installed package set
different from what your project uses.
The simplest solution to this problem is to launch your editor from within the nix-shell environment:
$ nix-shell
[nix-shell] $ code .
However, launching a nix-shell every time you want to edit a file is somewhat tedious, so an alternative is to use direnv. There are several solutions that will propagate information from a nix-shell to a direnv envrc file. You can then use a direnv support plugin in your editor (emacs has one, vscode has one) to get the right environment for the server launch.
Yet another solution is to use a plugin that loads the nix-shell directly in the editor, such as Nix Environment Selector for VSCode.
How to build a Haskell project using Stack¶
Stack is a popular build tool for Haskell
projects. It has first-class support for Nix. Stack can optionally use
Nix to automatically select the right version of GHC and other build
tools to build, test and execute apps in an existing project downloaded
from somewhere on the Internet. Pass the --nix
flag to any stack
command to do so, e.g.
git clone --recurse-submodules https://github.com/yesodweb/wai.git
cd wai
stack --nix build
If you want stack
to use Nix by default, you can add a nix
section to the stack.yaml
file, as explained in the Stack
documentation.
For example:
nix:
enable: true
packages: [pkgconfig zeromq zlib]
The example configuration snippet above tells Stack to create an ad hoc
environment for nix-shell
as in the below section, in which the
pkgconfig
, zeromq
and zlib
packages from Nixpkgs are
available. All stack
commands will implicitly be executed inside
this ad hoc environment.
Some projects have more sophisticated needs. For examples, some ad hoc
environments might need to expose Nixpkgs packages compiled in a certain
way, or with extra environment variables. In these cases, you’ll need a
shell
field instead of packages
:
nix:
enable: true
shell-file: shell.nix
For more on how to write a shell.nix
file see the below section.
You’ll need to express a derivation. Note that Nixpkgs ships with a
convenience wrapper function around mkDerivation
called
haskell.lib.buildStackProject
to help you create this derivation in
exactly the way Stack expects. However for this to work you need to
disable the sandbox, which you can do by using
--option sandbox relaxed
or --option sandbox false
to the Nix
command. All of the same inputs as mkDerivation
can be provided. For
example, to build a Stack project that including packages that link
against a version of the R library compiled with special options turned
on:
with (import <nixpkgs> { });
let R = pkgs.R.override { enableStrictBarrier = true; };
in
haskell.lib.buildStackProject {
name = "HaskellR";
buildInputs = [ R zeromq zlib ];
}
You can select a particular GHC version to compile with by setting the
ghc
attribute as an argument to buildStackProject
. Better yet,
let Stack choose what GHC version it wants based on the snapshot
specified in stack.yaml
(only works with Stack >= 1.1.3):
{nixpkgs ? import <nixpkgs> { }, ghc ? nixpkgs.ghc}:
with nixpkgs;
let R = pkgs.R.override { enableStrictBarrier = true; };
in
haskell.lib.buildStackProject {
name = "HaskellR";
buildInputs = [ R zeromq zlib ];
inherit ghc;
}
How to create ad hoc environments for nix-shell
¶
The easiest way to create an ad hoc development environment is to run
nix-shell
with the appropriate GHC environment given on the
command-line:
nix-shell -p "haskellPackages.ghcWithPackages (pkgs: with pkgs; [mtl pandoc])"
For more sophisticated use-cases, however, it’s more convenient to save
the desired configuration in a file called shell.nix
that looks like
this:
{ nixpkgs ? import <nixpkgs> {}, compiler ? "ghc7102" }:
let
inherit (nixpkgs) pkgs;
ghc = pkgs.haskell.packages.${compiler}.ghcWithPackages (ps: with ps; [
monad-par mtl
]);
in
pkgs.stdenv.mkDerivation {
name = "my-haskell-env-0";
buildInputs = [ ghc ];
shellHook = "eval $(egrep ^export ${ghc}/bin/ghc)";
}
Now run nix-shell
— or even nix-shell --pure
— to enter a shell
environment that has the appropriate compiler in $PATH
. If you use
--pure
, then add all other packages that your development
environment needs into the buildInputs
attribute. If you’d like to
switch to a different compiler version, then pass an appropriate
compiler
argument to the expression, i.e.
nix-shell --argstr compiler ghc784
.
If you need such an environment because you’d like to compile a Hackage
package outside of Nix — i.e. because you’re hacking on the latest
version from Git —, then the package set provides suitable nix-shell
environments for you already! Every Haskell package has an env
attribute that provides a shell environment suitable for compiling that
particular package. If you’d like to hack the lens
library, for
example, then you just have to check out the source code and enter the
appropriate environment:
$ cabal get lens-4.11 && cd lens-4.11
Downloading lens-4.11...
Unpacking to lens-4.11/
$ nix-shell "<nixpkgs>" -A haskellPackages.lens.env
[nix-shell:/tmp/lens-4.11]$
At point, you can run cabal configure
, cabal build
, and all the
other development commands. Note that you need cabal-install
installed in your $PATH
already to use it here — the nix-shell
environment does not provide it.
How to create Nix builds for your own private Haskell packages¶
If your own Haskell packages have build instructions for Cabal, then you
can convert those automatically into build instructions for Nix using
the cabal2nix
utility, which you can install into your profile by
running nix-env -i cabal2nix
.
How to build a stand-alone project¶
For example, let’s assume that you’re working on a private project
called foo
. To generate a Nix build expression for it, change into
the project’s top-level directory and run the command:
cabal2nix . > foo.nix
Then write the following snippet into a file called default.nix
:
{ nixpkgs ? import <nixpkgs> {}, compiler ? "ghc7102" }:
nixpkgs.pkgs.haskell.packages.${compiler}.callPackage ./foo.nix { }
Finally, store the following code in a file called shell.nix
:
{ nixpkgs ? import <nixpkgs> {}, compiler ? "ghc7102" }:
(import ./default.nix { inherit nixpkgs compiler; }).env
At this point, you can run nix-build
to have Nix compile your
project and install it into a Nix store path. The local directory will
contain a symlink called result
after nix-build
returns that
points into that location. Of course, passing the flag
--argstr compiler ghc763
allows switching the build to any version
of GHC currently supported.
Furthermore, you can call nix-shell
to enter an interactive
development environment in which you can use cabal configure
and
cabal build
to develop your code. That environment will
automatically contain a proper GHC derivation with all the required
libraries registered as well as all the system-level libraries your
package might need.
If your package does not depend on any system-level libraries, then it’s sufficient to run
nix-shell --command "cabal configure"
once to set up your build. cabal-install
determines the absolute
paths to all resources required for the build and writes them into a
config file in the dist/
directory. Once that’s done, you can run
cabal build
and any other command for that project even outside of
the nix-shell
environment. This feature is particularly nice for
those of us who like to edit their code with an IDE, like Emacs’
haskell-mode
, because it’s not necessary to start Emacs inside of
nix-shell just to make it find out the necessary settings for building
the project; cabal-install
has already done that for us.
If you want to do some quick-and-dirty hacking and don’t want to bother
setting up a default.nix
and shell.nix
file manually, then you
can use the --shell
flag offered by cabal2nix
to have it
generate a stand-alone nix-shell
environment for you. With that
feature, running
cabal2nix --shell . > shell.nix
nix-shell --command "cabal configure"
is usually enough to set up a build environment for any given Haskell
package. You can even use that generated file to run nix-build
, too:
nix-build shell.nix
How to build projects that depend on each other¶
If you have multiple private Haskell packages that depend on each other,
then you’ll have to register those packages in the Nixpkgs set to make
them visible for the dependency resolution performed by callPackage
.
First of all, change into each of your projects top-level directories
and generate a default.nix
file with cabal2nix
:
cd ~/src/foo && cabal2nix . > default.nix
cd ~/src/bar && cabal2nix . > default.nix
Then edit your ~/.config/nixpkgs/config.nix
file to register those
builds in the default Haskell package set:
{
packageOverrides = super:
{
haskellPackages = super.haskellPackages.override {
overrides = self: super: {
foo = self.callPackage ../src/foo {};
bar = self.callPackage ../src/bar {};
};
};
};
}
Once that’s accomplished, nix-env -f "<nixpkgs>" -qA haskellPackages
will show your packages like any other package from Hackage, and you can
build them
nix-build "<nixpkgs>" -A haskellPackages.foo
or enter an interactive shell environment suitable for building them:
nix-shell "<nixpkgs>" -A haskellPackages.bar.env